BLOOD AND BLOOD-VESSELS OF YOLK-SAC 5 



instance, and others are of opinion that the leucocytes found in the wall of the gut are 

 derived from the entodermic epithelium ; while, again, there is some reason to believe that 

 ectoderm-cells may wander from the neural crest, and even from the neural canal along the 

 primitive ventral nerve-roots, and spread to quite distant parts within the mesoderm. 



In connexion with the first appearance of individual cells between the layers, it may here 

 be mentioned that, according to the observations of Szily, 1 the epithelia are all connected together 

 by protoplasmic threads which are spun out as they draw apart in the course of development. 

 The spaces between the epithelial layers are not vacant, as they appear to be in sections 

 treated by ordinary methods, but are occupied by a delicate protoplasmic reticulum, which 

 forms a basis on which the wandering mesenchyme-cells arrange themselves into a syncytium. 



The blood and blood-vessels first appear in the wall of the yolk-sac. In 

 the lower mammals a vascular area is developed (fig. 85), as in the Sauropsida, 



S'ttS? 



SSs***. . 



FIG. 84. TRANSVERSE SECTION THROUGH THE TRUNK OF A RABBIT EMBRYO OF THE 



ELEVENTH DAY. (T. H. BryCC.) 



in. p., muscle-plate ; x.p., skin-plate ; sc, sclerotome ; w.d., Wolffian duct ; iv.t., Wolffian tubule. The 

 rirlge in which the duct and tubule lie is the Wolffian ridge. To the left the section has cut the wall 

 of a Woffian tubule where it is connected by a cellular strand with the coelomic epithelium. A, aorta ; 

 C, ccelom ; UV, UV, umbilical veins. 



but in the Primates the earliest vessels appear on the under aspect of the sac 

 (fig. 79) and gradually extend over its upper pole, until the whole sphere is 

 covered by a vascular network. Further, in Primates there is no terminal sinus. 

 These are regarded as secondary modifications due to the small size of the 

 yolk-sac. 



The first indication of blood and blood-vessels is the appearance of irregular 

 projections on the surface of the vesicle due to the formation of the blood-islands 

 of Pander between the entoderm and mesoderm. The blood-islands are groups of 

 rounded nucleated corpuscles closely packed together : indeed the cell outlines are 



1 Anat. Anzeiger, xxiv. 1903. 



